The more primitive algae were probably less efficient in producing oyxgen. It would take time to get that system right especially as oxygen is a deadly poison to anaerobic lifeforms. And the earliest lifeforms were anaerobic. Or at least evolved from anaerobic lifeforms.

Sure, but the point is, even speeding it up a million times is thousands of years.

The real bottleneck is that life creates wholesome conditions for life. Small amounts of biomass have small slow effects. It is the cumulative effects that make the big changes. The built up is incredibly slow, but once the tipping point is hit, change is rappid and profound. To speed up terraforming, you've got to get to the tipping point faster. Which is the vast problem.

Well, realistic timescales wind up not being very interesting. I mean, it took about two billion years for Earth to go from the initial evolution of blue-green algae to having a breathable atmosphere.

Yeah, but to be fair, it's entirely plausible that that process could be speeded up substantially by artificial methods. Not by us today, of course, but it could probably be done.

Of course 'speeded up' might easily mean converting 2 billion into 2 million or 20,000. Still a long time on a human scale.

Also, Earth 4 billion years ago was already a better terraforming candidate than Venus or Mars today. A lot of how hard terraforming is depends on how close your target world is to what you want. An interstellar society would have far more potential 'targets' to choose from.

Yeah, but to be fair, it's entirely plausible that that process could be speeded up substantially by artificial methods. Not by us today, of course, but it could probably be done.

Of course 'speeded up' might easily mean converting 2 billion into 2 million or 20,000. Still a long time on a human scale.

Also, Earth 4 billion years ago was already a better terraforming candidate than Venus or Mars today. A lot of how hard terraforming is depends on how close your target world is to what you want. An interstellar society would have far more potential 'targets' to choose from.

The terraformation of a planet like Mars would be a giveaway that the solar system had been a species original home, or some has scary high tech.